The present invention relates to a light diffusing sheet material having utility in various fields, including that of enhancing video displays, such as LCD displays.
In liquid crystal displays the bright image seen by the viewer is generated through the liquid cell either by light generated within the assembly by a back light or by the use of ambient light which is first transmitted through the cell and reflected at the rear of the cell, re-emerging through the cell to create a bright image. In this latter arrangement, due to the position of the viewer, the majority of light must be accepted off axis, preferably should not be further diffused on entering the display, and should preferably re-emerge on axis with some additional diffusion in order to provide an acceptable viewing cone. It is well known that Fresnel-like structures have the ability to redirect light; as circular arrangements of facets to create lenses, or as linear arrangements of facets to create off axis effects. Structures of these types have been proposed incorporating materials with light diffusing characteristics, as described in U.S. Pat. No. 4,911,529, where the diffusive effect is provided by a so-called bulk diffuser comprising small particles of a material of a first refractive index dispersed in a matrix material of a second refractive index. Such materials are described in, for example U.S. Pat. No. 4,983,016, EP-A-0464499 or EP-A-0843203. An important characteristic of such bulk diff-user materials is that the amount of diffusion taking place within a given composition is dependent on the thickness of material through which the light travels. As a result, light entering an assembly containing such material off axis becomes more diffuse and reduced in intensity than light which enters on axis.
According to the present invention there is provided an optical device comprising an element of light-transmitting material having a surface thereof configured to form a stepped, Fresnel-type light refracting element, such light transmitting material incorporating an array of integral graded refractive index structures adapted to impart light dispersing or diff-using characteristics to said light-transmitting sheet material or the element having a layer configured to form said stepped surface and an additional layer incorporating such an array of integral graded refractive index structures.
In certain instances, it may be desirable to render one surface of said element reflecting or partially reflecting by coating with a material with the necessary optical properties, such as a metal applied by deposition or sputtering.
The sheet material may comprise, for example, a photopolymer, in which the graded refractive index features are formed by exposure of a precursor of the material, (such as a monomer) to appropriate radiation in a predetermined pattern so as to produce localised variations of light intensity within the material and hence localised variations in degree of polymerisation and thus in refractive index. As another example, the material may be dichromated gelatine (DCG), in which the desired graded refractive index features are formed by exposure of the material itself to such a radiation pattern, to produce an equivalent effect. Such materials, correctly processed, possess the previously described desirable optical characteristics. The process techniques used may, for example include, those disclosed in EP-0294122 or EP-0801767 or U.S. Pat. No. 5,695,895 or GB-A-1499135. These types of diffusing materials may themselves have off axis or asymmetric characteristics, as described in EP 0768565, which can further enhance the off axis effects generated by the Fresnel structure. Methods of producing sheets of light diffusing material in which the light-diffusing characteristics are due to refractive index variations within the material are known and will not be discussed in greater detail here. These known methods, however, are directed to the production of either planar light-diffusing sheets or light-diffusing sheets having a surface configuration in the form of a plurality of convex domes, for example, adapted to enhance the diffusive characteristics of the material.